The present invention relates to flow meters, and in particular to a positive displacement flow meter for a spray coating system, which may rapidly be flushed clean of coating material.
Color change systems for spray coating apparatus have particular application in industrial operations where articles are to be spray coated at a spray station or as they move along a production line. Color change systems provide for a wide variety of colors to be sprayed from a single spray gun. With many conventional systems, a plurality of supply containers of coating material, each of a different color and having a separate transfer pump or a source of pressurization for the container, are connected with a manifold of a color changer through valve controlled ports. An outlet from the manifold is coupled to an inlet to a spray gun, and to spray material of a particular color the manifold port valve associated therewith is opened for flow of the material through the manifold to the gun. After completion of spraying material of a particular color, the manifold, gun and connecting line are cleaned with a flushing media, which usually comprises alternate bursts of relatively high velocity solvent and compressed air, to prepare the system for spraying material of a different color.
In many spray coating applications, it is necessary that the quantities of coating materials supplied to the spraying equipment be accurately metered or measured to supply a specified amount of material to be applied onto an article. For the purpose, positive displacement flow meters are often used in line with the outlet from the color changer. Such flow meters may have a pair of sensing elements in the form of meshed gears that are rotated by and at a rate in accordance with the volume flow rate of coating material through the flow meter. The rate of rotation of one of the gears or the rotations made by it is detected and used to provide an indication of the flow rate and/or total volume flow of coating material to the spray apparatus.
Conventional positive displacement flow meters used in paint spraying operations are prone to failure when cleaned with alternate bursts of relatively high velocity air and solvent. The flow meters are designed for specific flow ranges, and have limits on the internal operating speeds of their gears or sensing elements. When such a flow meter is subjected to a flow of relatively high velocity air during flushing, the air causes overdriving or too high a speed of rotation of the sensing elements, which can result in damage to and failure of the flow meter. A second failure mode occurs as a consequence of solvent being injected into the flow meter following the air. When the liquid solvent hits the overdriven sensing elements, the impact can cause severe stress and failure of the flow meter.